1. Field of the Invention
The present invention relates, in general, to reinforcing steel cords for a variety of rubber products, such as tires and conveyor belts, and, more particularly, to a reinforcing steel cord formed by twisting a plurality of external element wires around a twisted flat core, thus having a plurality of interspaces between the core and the wires in addition to a plurality of interspaces between the wires, the steel cord thus allowing the rubber material to be more effectively penetrated into the cord through the interspaces during a production process of the rubber products and being free from an undesirable movement of the core within the cord, and being improved in its ageing adhesive force with the rubber material, the present invention also relating to a method and device for producing such steel cords.
2. Description of the Prior Art
As well known to those skilled in the art, steel cords are used as reinforcing materials for rubber products or elastomer products, such as tires or conveyor belts. The steel cords, used as the reinforcing materials for such rubber products, are superior in desired characteristics, such as strength, modulus, heat resistance and fatigue resistance, in comparison with other conventional reinforcing materials, such as organic or inorganic fibers. Therefore, the steel cords have been more preferably used as reinforcing materials of such rubber products than such other reinforcing materials. Particularly when such steel cords are used as the material of the carcass or the steel belt layer of a radial tire, the steel cords remarkably improve the fretting resistance, durability and steering response of the tire.
A steel cord, used as a reinforcing material for radial tires or conveyor belts, is typically formed by twisting a plurality of element wires together to form a strand structure or by twisting a plurality of strands together to form a wire rope structure. In order to allow such steel cords to perform a desired reinforcing function within a rubber product, it is necessary for the steel cords to be physically, chemically and firmly integrated with the rubber material.
FIGS. 1 to 5 are sectional views, respectively showing examples of conventional reinforcing steel cords for radial tires.
FIG. 1 shows a steel cord, which has a double layer twisted structure and is typically used as the belt layer material of steel belted radial tires for large-scaled vehicles, such as trucks or buses. AS shown in the drawing, the steel cord 1 has a 3+6 element wire structure wherein six external element wires 1b are twisted around a core to form a double layer twisted structure, with the core being formed by twisting three core element wires la together to form a core.
However, the above double layer twisted steel cord 1 is problematic in that it is somewhat complex in structure since it has many element wires. In addition, the above steel cord 1 has to be produced through two twisting processes, or a primary twisting process of twisting the three core element wires la to form a core and a second twisting process of twisting the six external element wires 1b around the core to form a cord. This finally complicates the process of producing the reinforcing steel cords in addition to an increase in the production cost of the steel cords. In the above steel cord 1, a central space H is formed at the center of the three twisted core element wires 1a, but it is almost impossible for the rubber material to be penetrated into the central space H during a tire production process. This steel cord 1 is thus undesirably reduced in its ageing adhesive force with the rubber material.
Another problem experienced in the above steel cord 1 resides in that the cord 1 is somewhat heavy and has a large diameter, thus being not agreeable with the recent trend of lightness of tires or of an improvement in maximum safe mileage.
In an effort to overcome the above-mentioned problems of the double layer twisted steel cord 1, a steel cord, having a single layer twisted open structure, has been proposed as disclosed in Japanese Laid-open Publication No. Heisei. 6-65,877. This Japanese steel cord, shown in FIG. 2 of the accompanying drawings, has a small diameter in addition to a simple construction. This steel cord is also produced through a single process free from the primary twisting step of forming the twisted core different from the steel cord 1 of FIG. 1. As shown in the drawing, a plurality of element wires, for example, six element wires 2a are twisted together to form a steel cord 2 while being respectively and exceedingly preformed. This steel cord 2 is, thereafter, externally forced to be somewhat flattened, thus having a generally elliptical cross-section. In the above steel cord 2, a plurality of interspaces S are formed between the element wires 2a. 
The above steel cord 2 is produced through a single twisting process, thus simplifying the cord production process in addition to a reduction in the cord production cost. In the above steel cord 2, the element wires 2a are somewhat loosely integrated since they are respectively and exceedingly preformed during the process of producing the cord 2, thus forming the desired interspaces S between the wires 2a. Due to such interspaces S, the rubber material is allowed to be penetrated into the steel cord 2 during a process of producing a steel belted radial tire. In addition, since each flat surface of the above steel cord 2 is almost kept on the same plane within the total length of the cord 2, it is possible to reduce the thickness of a resulting tire while preferably reducing the weight of the tire.
However, the above steel cord 2 is problematic in that since the element wires 2a are loosely twisted together while being respectively preformed, the cord 2 is exceedingly high in its elongation even in the case of application of low load. This cord 2 is thus difficult to be handled by a worker during a tire production process. In order to preform the element wires 2a within a predetermined range, it is necessary to mechanically process the element wires 2a using a specific preforming jig, such as a plate-type preforming device or a rotary-type preforming device. In such a case, severe friction is generated at the contact portions between the element wires 2a and the jig, thus undesirably removing brass coating layers from the surfaces of the element wires 2a and damaging the wires 2a. This finally reduces the rubber adhesive force and buckling fatigue resistance of the steel cord 2.
Particularly, the above steel cord 2 is very difficult to handle during a process of producing desired rubber products having the cords 2 and necessarily has a fine difference in the low load elongation between the wires 2a. Therefore, it is difficult to regularly array the steel cords 2 within a topping sheet, thus resulting in irregular quality of resulting topping sheets. In the case of tires using belt layers made of such steel cords 2, the steel cords 2 may be easily loosened during a rotation of the tires on a street. This may finally allow the belt layers to be unexpectedly deformed, thus reducing the steering response of the tires and occasionally causing safety hazards.
FIG. 3 shows a conventional steel cord 3, which has a 1+6 element wire structure wherein six external element wires 3b are twisted around a core 3a, made of one core element wire having a circular cross-section, to form a cord. On the other hand, FIG. 4 shows another conventional steel cord 4, which has a 1+6 element wire structure, with six external element wires 4b being twisted around a core 4a, made of one core element wire, to form a steel cord in the same manner as that described for the cord 3 of FIG. 3. However, the core 4b of this steel cord 4 is rolled by a press roll pair to have a flat cross-section different from the that of the cord 3.
In the steel cords 3 and 4 each having a 1+6 element wire structure of FIGS. 3 and 4, the structural stability of the cords 3 and 4 is improved due to the cores 3a and 4a. It is also possible to reduce the elongation when the cords 3 and 4 are stretched. However, the above steel cords 3 and 4 are problematic in that it is very difficult for the rubber material to penetrate into the junctions between the cores 3a, 4a and the external element wires 3b, 4b since the external element wires are densely twisted around the core while being brought into continuous linear contact with the core.
In a steel belted radial tire having a belt layer consisting of the above reinforcing steel cords 3 or 4, the steel belt layer repeats a buckling action during a rotating action of the tire on a street, thus being repeatedly tensioned, compressed and thereby severely pressurized. Due to such a buckling action of the steel belt layer, the neighboring element wires 3a and 3b, 4a and 4b of each steel cord are brought into frictional contact with each other, thus being gradually fretted at their frictional contact surfaces and being frictionally fatigued at the surfaces. This may finally cause a breakage of some steel cords within the belt layer.
Another problem of the above steel cords 3 and 4 resides in that the core 3a or 4a fails to be integrated with external element wires 3b or 4b by the rubber material, but is freely kept within the central space defined by the twisted external wires 3b or 4b. Therefore, each of the steel cords 3 and 4 undesirably results in a core migration wherein the core 3a or 4a moves to the edge of the belt layer.
FIG. 5 is a sectional view of a conventional steel cord 5, which has a 1+6 element wire structure with some external element wires being partially preformed to overcome the above-mentioned problems experienced in the steel cords 3 and 4 of FIGS. 3 and 4. In the steel cord 5 of FIG. 5, some external element wires 5bxe2x80x2, twisted around the core 5a to form a cord, are preformed, and so the junctions between the preformed element wires 5bxe2x80x2 are partially open, thus improving penetration of the rubber material into the steel cord 5.
However, the above steel cord 5 has the following problems. That is, the steel belt layer consisting of such steel cords 5 repeats a buckling action during a continuous rotating action of a tire on a street, thus being repeatedly tensioned, compressed and thereby instantaneously and severely impacted. In such a case, the steel cords 5 within the steel belt layer are overloaded. Therefore, the tensile force and the compression force are concentrated on the non-preformed external wires 5b, having a low preforming ratio or being low in the supplied element wire length per unit length of the steel belt layer. The above steel cord 5 is thus inferior in structural stability.
In order to preform the element wires 5bxe2x80x2 within a predetermined range, it is necessary to mechanically process the element wires 5bxe2x80x2 using a specific preforming jig, such as a toothed gear. In such a case, a severe friction is generated at the contact portions between the element wires 5bxe2x80x2 and the jig, thus undesirably removing brass coating layers from the surfaces of the element wires 5bxe2x80x2 and damaging the wires 5bxe2x80x2. This finally reduces the rubber adhesive force and buckling fatigue resistance of the steel cord 5.
Accordingly, the present invention has been made keeping in mind the above problems occurring in the prior art, and an object of the present invention is to provide a reinforcing steel cord for rubber products, which is formed by twisting a plurality of external element wires around a core, the core being improved in its structure so as to minimize the continuous contact area between the core and external element wires and to form desired interspaces between the core and the wires in addition to a plurality of interspaces between the wires, thus allowing the rubber material to be more effectively penetrated into and filled in the interspaces during a production process of the rubber products and being improved in its ageing adhesive force with the rubber material.
Another object of the present invention is to provide a reinforcing steel cord for rubber products, which is very firmly integrated with the rubber material of a desired rubber product, thus having a low elongation in the case of application of low load and being improved in its structural stability, and which effectively resists any buckling action and is improved in its fretting resistance while being free from damaging the brass coating layers of the element wires, and which completely eliminates the problem of a core migration.
A further object of the present invention is to provide a method and device for producing such reinforcing steel cords.
In order to accomplish the above objects, the present invention provides a reinforcing steel cord for rubber products, which is formed by twisting a plurality of brass coated external element wires around a flat and spirally twisted core, with the twisted direction of the core being the same as or opposite to that of the resulting steel cord.
In the steel cord of this invention, the flat core is twisted in the same direction as that of the resulting cord or in a direction opposite to that of the cord, with the pitch of the core being 0.2 to 2 times the pitch of the cord. Therefore, it is possible to form a plurality of desired interspaces between the core and the external wires and between the external wires within the steel cord.
In an embodiment of this invention, at least one of the external wires, twisted around the flat and spirally twisted core to the cord, has a flat cross-section in addition to a spirally twisted structure in the same manner as that of the core.
The above-mentioned interspaces within the steel cord of this invention improve the rubber penetration into the cord, thus allowing the rubber material to be completely filled in the cord. The rubber material filled in the interspaces almost completely prevents the core from coming into direct contact with the external wires in addition to a prevention of contact between the external wires.
Therefore, when such steel cords are set within a steel belted radial tire through a vulcanizing process, the rubber material effectively penetrates into the cords through the open interspaces and is filled in the cords. This effectively prevents an abrasion of the cords caused by the friction contact between the wires and minimizes an undesirable breakage of the wires within the steel cords. The rubber material within the interspaces around the core firmly holds the position of the core within the cord, thus eliminating the problem of a core migration. The steel cord of this invention is produced without using a conventional preforming device undesirably scratching or damaging the brass coated surfaces of the steel wires, thus improving the rubber adhesive force of the steel cord in addition to an improvement in ageing adhesion of the cord with the rubber material.
The steel cord of this invention has a 1+n element wire structure (xe2x80x9cnxe2x80x9d representing the number of external element wires). In the steel cord, it is preferable to set the number xe2x80x9cnxe2x80x9d to one of three to nine. The above steel cord, having a first multi-layer twisted structure, may be directly used as a reinforcing material for rubber products or may be used as a core of another steel cord having a second multi-layer twisted structure. The above steel cord, having the first multi-layer twisted structure, may be also used as strands of another steel cord having a closing structure, which has been typically expressed using the symbol xe2x80x9cxxe2x80x9d in the art. Alternatively, the steel cord of this invention may have a combined structure, having both such a closing structure and a multi-layer twisted structure. Such a multi-layer twisted structure has been typically expressed using the symbol xe2x80x9c+xe2x80x9d in the art.
The above-mentioned reinforcing steel cord for rubber products is produced through the steps of cold-rolling a brass-plated steel wire having a circular cross-section to give the steel wire a flat cross-section, axially and spirally twisting the steel wire around an axis of the wire, thus forming a desired flat and spirally twisted core, and twisting a plurality of external wires around the core to form a desired twisted steel cord in a way such that at least one of the twisted direction and the pitch of the twisted core is different from that of the twisted cord.